This invention is in the field of liquid spark-ignition engine fuel octane testing. More specifically, embodiments of the invention are directed to methods of measuring the octane number of a sample fuel by running that fuel on a test engine, for example an ASTM standard Cooperative Fuels Research (CFR) engine.
Gasoline fuels for use in internal combustion engines are sold according to their octane ratings, with fuels of higher octane ratings priced higher than those with lower octane ratings. As known in the industry, the octane rating (or “octane number”) is a measure of the ability of the fuel to resist “detonation” which is the spontaneous combustion of unburned gasoline in the cylinder triggered by the high temperature and pressure resulting from the spark-initiated combustion, but occurring well after that combustion and near the end of the engine cycle. This detonation, which is also referred to as “knock” or “ping” due to its characteristic resonant sound, adds nothing to the power output of the engine, but can cause engine damage over time. In a practical sense, a higher octane fuel will resist detonation as compared with lower octane fuel, under the same conditions of temperature and pressure.
As known in the industry, the conventional method for measuring the octane rating of a gasoline fuel is to operate a standardized Cooperative Fuels Research (CFR) engine under conditions specified by industry standards (e.g., ASTM standards D2699-12 and D2700-12, incorporated herein by reference). An example of a modern CFR engine is the Waukesha CFR Combination Research and Motor Method Octane Rating Unit available from General Electric. CFR engines conventionally include a detonation pickup device, such as a magnetostrictive sensor, that generates an electrical signal that is proportional to the time rate-of-change of combustion chamber pressure in the engine while being run on a sample of the fuel under evaluation.
FIG. 1 illustrates a conventional CFR engine with the sensors involved in the conventional determination of KI for a fuel under evaluation. CFR engine 2 includes cylinder 4 within which piston 5 is disposed; piston 5 is mechanically coupled to crank 7 in the conventional manner. Fuel and air are mixed at carburetor 6 in the conventional manner, with the fuel received via an adjustable fuel supply 8, and intake air conditioned by intake air dehumidifier 9 and intake air heater 11. The fuel-air mixture is further heated by intake mixture air heater 13 and introduced into cylinder 4 at cylinder head 10 via the appropriate valve, near spark plug 14 that is also disposed at cylinder head 10. Exhaust gases from combustion are expelled from cylinder 4 via exhaust system 15. Starter motor 16 is provided to maintain a constant rotational speed of the engine system at the standard speed of 600 RPM (for the Research Octane Number, or RON, method) or 900 RPM (for the Motor Octane Number, or MON, method).
For purposes of evaluating the octane rating of fuels, detonation pickup 17 is mounted to the wall or head chamber of cylinder 4 of CFR engine 2, as shown in FIG. 1. Detonation pickup 17 is conventionally implemented as a magnetostrictive pressure rate-of-change transducer, which generates an electrical signal that is proportional to the time rate-of-change of combustion chamber pressure in cylinder 4. The height of cylinder head 10 of CFR engine 2 in this example is adjustable, which allows adjustment of the compression ratio of engine 2 and, indirectly, the nominal pressure inside cylinder 4. In addition, adjustable fuel supply 8 allows control of the air-fuel ratio supplied to engine 2. To evaluate a fuel sample, a control variable (typically the air-fuel ratio, or AFR) is varied while engine 2 is running, with detonation pickup 17 sensing knock events over that time interval. The industry standard test methods identify the peak time-rate-of-change of pressure as sensed by detonation pickup 17 as the peak knock intensity (KI), from which the octane rating of the fuel is determined. Typically, the KI of the fuel under evaluation is compared against the KIs of two primary reference fuels (PRFs), one PRF having a known high octane and the other PRF having a known low octane. The octane rating of the fuel under evaluation is typically calculated by linear interpolation between the octane ratings of the two PRFs, according to the relative KI of the fuel under evaluation relative to the KI values of the two PRFs.
Conventionally, other measurements besides the AFR, the height of cylinder head 10, and the output of detonation pickup 17 are also acquired at the standardized engine. As shown in FIG. 1 in connection with CFR engine 2, temperature sensor 18a senses the temperature of the heated intake air entering carburetor 6, temperature sensor 18m senses the temperature of the heated fuel-air mixture entering cylinder head 10, and barometer 19 measures the barometric pressure at CFR engine 2. As described by way of example in the ASTM MON Standard (D2700-12), these temperature measurements are conventionally used as feedback in the temperature control loop to maintain a constant temperature, and the barometric pressure measurement is used so as to adjust the cylinder head height to a normalized compression ratio (i.e., normalized to sea level).
As known in the industry, the price of gasoline varies with its octane rating, with higher octane rated-fuel commanding a higher price at the pump. Gasoline product as sold at a particular octane rating is typically blended to exhibit at least that octane rating when evaluated by a CFR engine such as that of FIG. 1. To the extent that error is present in the CRF test process, sufficient margin in the octane rating of the blended gasoline must be provided to ensure that all of the output meets the rated octane level. However, over-blending (i.e., including more higher octane fuel in the blend than necessary for the octane rating at which it is sold) to provide this margin adds significant cost that will not be recovered in the price paid to the refiner, and is therefore not economically optimal.